Hydrocarbon mixtures containing significant quantities of light olefins are frequently encountered in petroleum refineries, particularly as a byproduct of fluidized catalytic cracking (FCC) processes. Because of the ease with which olefins react, these streams serve as intermediate feedstocks in a variety of hydrocarbon conversion processes. Many olefinic conversion processes require that the olefinic feed be provided in a highly purified condition. However, processes which may utilize the olefinic feedstocks without the need for further separation and purification are highly desirable.
Although the main purpose of fluidized catalytic cracking is to convert gas oils to compounds of lower molecular weight in the gasoline and middle distillate boiling ranges, significant quantities of C.sub.1 -C.sub.4 hydrocarbons are also produced. These light hydrocarbon gases are rich in olefins, which are useful for conversion to gasoline blending stocks by means of polymerization and/or alkylation.
Fractionation of effluent from the fluid catalytic cracking reactor has been employed to effect an initial separation of this stream. The gaseous overhead from the main fractionator is collected and processed in the FCC unsaturated gas plant (USGP). Typically, the gases are compressed, contacted with a naphtha stream, scrubbed with an amine solution to remove acidic sulfur components, and then fractionated to provide light olefins and isobutane for alkylation, light olefins for polymerization, n-butane for gasoline blending and propane for LPG. Ethane and other light gases are usually recovered for use as fuel.
Since alkylation units were more costly to build and operate than polymerization units, olefin polymerization was initially favored as the route for providing blending stocks. Increased gasoline demand and rising octane requirements soon favored the use of alkylation because it provided gasoline blending stocks at a higher yield and with a higher octane rating than the comparable polymerized product. However, catalytic alkylation can present some safety and disposal problems. In addition, feedstock purification is required to prevent catalyst contamination and excess catalyst comsumption. Further, sometimes there is insufficient isobutane available in a refinery to permit all the C.sub.3 -C.sub.4 olefins from the FCC to be catalytically alkylated.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of ZSM-5 or related zeolite. In U.S. Pat. Nos. 4,150,062 and 4,227,992 Garwood et al disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3 + olefins. An economic fluid bed process, sometimes known as MOG, is especially useful in upgrading mixed light gas feedstreams containing olefins in mixture with other FCC light cracking gas components. The MOG process is disclosed by Avidan et al in U.S. Pat. application Ser. No. 006,407, filed 23 Jan 1988, incorporated herein by reference.
The process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization reaction may be followed by other reactions, such as cyclization to form aromatics. Using an acid crystalline metallosilicate zeolite, such as ZSM-5 or related shape-selective catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. In a preferred fluidized bed gasoline operating mode reactor system, ethylene and the other lower olefins are catalytically oligomerized at elevated temperature and moderate pressure. Under these conditions ethylene conversion rate is greatly increased and lower olefin oligomerization is nearly complete to produce an olefinic gasoline comprising hexene, heptene, octene and other C.sub.5 + hydrocarbons in good yield. Other C.sub.5+ products include aromatics, naphthenes and paraffins. Such a conversion unit has a significant alkane-rich C.sub.1 -C.sub.4 - aliphatic hydrocarbon byproduct, comprising n-butanes, i-butanes, propane, ethane and minor amounts of unreacted lower olefins.
U. S. Pat. Nos. 4,012,455 and 4,090,949 (Owen and Venuto) and published European Patent Application No. 0,113,180 (Graven and McGovern) disclose integration of olefins upgrading with a typical FCC plant. In the EPA application the olefin feedstock for MOGD comprises the discharge stream from the final stage of the wet gas compressor or the overhead from the high pressure receiver which separates the condensed effluent from the final stage wet gas compressor contained in the gas plant. The present invention improves upon such integrated processes by incorporating olefins upgrading advantageously with the FCC reactor and gas plant, providing a novel use for alkane-rich byproduct of the olefin upgrading unit.